Which diagram shows the correct distribution of electrons in the electron shells of a magnesium atom? A gray circle labeled 3 P 4 N is surrounded by two concentric circles. The inner circle has gray dots at the north and south positions, and the outer circle has a dot at the north position. A gray circle labeled 4 P 5 N is surrounded by two concentric circles. Each of the concentric circles has gray dots at the north and south positions. A gray circle labeled 12 P 12 N is surrounded by three concentric circles. The innermost circle has gray dots at the north and south positions; the middle circle has eight gray dots equally spaced around the circle, with dots aligned with those in the inner circle; the outer circle has gray dots at the north and south positions. A gray circle labeled 12 P 12 N is surrounded by three concentric circles. The innermost circle has gray dots at the north and south positions; the middle circle has eight gray dots equally spaced around the circle, with dots aligned with those in the inner circle; the outer circle has a gray dot at the north position.